Device for melting or purifying of inorganic sustances

ABSTRACT

The invention relates to a device for the melting or purifying of inorganic substances, in particular of glass, which comprises a number of metal tubes which may be attached to a cooling medium and which are arranged next to each other, in such a way that together they form a container, a high frequency coil for the injection of energy into the container contents and a plastic coating for the metal tubes, the decomposition temperature of which lies below the temperature of the melt. The cooling system is configured and arranged, such that the temperature of the boundary layer of the melt, immediately surrounding the component, lies beneath that of the decomposition temperature of the coating material.

[0001] The invention involves a device for melting or refining glass orglass ceramics.

[0002] Devices of this type have become known in the form of so-calledskull crucibles. They contain a crucible wall. This wall is generallycylindrical. It is made out of a ring of vertical metal pipes. Slitsremain between pipes that are adjacent to each other. The bottom of thecrucible can also be made out of metal pipes. It can, however, also bemade out of a fireproof material. At their ends, they are connected tovertical pipes for supplying coolant and/or for discharging coolant.

[0003] The heating is done by an induction coil that surrounds thecrucible wall, and via which high-frequency energy can be introducedinto the contents of the crucible.

[0004] A skull crucible of this type has been made known, for example,from the patent EP 0 528 025 B1.

[0005] A skull crucible operates as follows: the crucible is filled witha glass batch or refuse glass (shards) or a mixture of them. The glass,and/or the melt, must be preheated at first in order to reach a certainminimum conductibility. The pre-heating is frequently done by burnerheating. Once the coupling temperature has been reached, then theadditional supply of energy via the irradiation of high-frequency energycan occur. Also, during the operation, it can be advantageous in specialcases to heat the melt by burners which act on the melt from above, orby hot exhaust gases, in addition to the heating by high-frequencyenergy. This is especially necessary in some cases when using a skullcrucible for refining. If the surface layer is namely cold and has acorrespondingly higher viscosity, then bubbles are prevented fromemerging out of the melt, or foam can form.

[0006] The skull principle can also be applied for containers other thancrucibles. See the patent DE 199 39 782 A1, for example. In thisprocess, several pipes are designed in a U-shape and lie adjacent toeach other, so that they form a cage-like skull channel with each otherthat is open to the top. An induction coil then surrounds this channelin such a way that winding sections extend along the side walls of thechannel.

[0007] The invention thus involves any type of device in which acontainer is made from many metal pipes, which themselves can beconnected to a cooling medium, and in which high-frequency energy isintroduced into the contents of the container.

[0008] The metal pipes mentioned are generally made of copper. Thisresults in the following problems:

[0009] On the one hand, there is the danger that copper from the copperpipes gets into the melt. This leads to pronounced color streaking as aresult of the copper impurity.

[0010] An additional problem consists in that gaseous or solidcomponents escaping from the glass, such as, for example: HF, P₂O₅,B₂O₃, SO₂, SO₃, Cl₂ precipitate onto the cooled part of the crucible notcovered by the melt and cause corrosion there. This leads to a damage ofthe crucible and to impurity of the melt.

[0011] Furthermore, a removal of the residual glass when cleaning thecontainer is expensive and painstaking, since this residual glasspersistently adheres to the surfaces of the metal pipes.

[0012] The purpose of the invention is to create a device of the typenamed at the beginning in such a way that impurities of the glass meltdue to the material of the metal pipe do not occur, that no corrosionoccurs, and that the problem of the adherence of the glass melt does notoccur.

[0013] This purpose is achieved by the characteristics of claim 1.

[0014] To achieve the intended purpose, different methods had beenproposed previously. The use of platinum instead of copper is not onlyconsiderably more expensive, but also is not a perfect solution in termsof preventing impurities in the glass melt. The concept of usingplastics had to be considered to be wrong from the beginning on, becauseof the low melting point of these materials. The inventors recognized,however, that plastics, in spite of their low softening temperature andin spite of their low decomposition temperature, come into considerationvery well as a coating material, and they are suitable for solving theproblems. As has been revealed in experiments, the cooling of the metalpipes caused the contact temperature between, on the one hand, theplastic of the plastic coating according to the invention, and on theother hand, the glass melt, to stay below the decomposition temperatureof the plastic. The plastic coating remained undamaged after theexperiments. The melting container could be reused.

[0015] By the plastic coating according to the invention, the mostimportant of the partial purposes named above is achieved. Namely, acontamination of the melt does not occur. In the practical case, alkalizinc silicate glasses were melted in the skull crucible. Fibers, whichwere manufactured out of this glass, had an extremely low damping, whichmeans that the coating prevented copper from diffusing out of the skullcrucible into the melt. In a control experiment, uncoated copper pipeswere used. These pipes led to considerable color streaking by copperimpurity.

[0016] In another experiment, phosphate and fluorophosphate glasses weremelted in a crucible designed according to the invention. The glassesexhibited a high optical purity, which otherwise is only obtained inplatinum melting crucibles. Furthermore, the melt was free of platinumparticles, which can, of course, not be guaranteed for platinumcrucibles. The glasses were suitable for high-energy laser applications.Devices with plastic coating according to the invention are alsosuitable for oxide salt or metal melts. In the process, the expert willmatch the parameters of cooling, on the one hand, and the plasticproperties, on the other hand, to each other.

[0017] The other important partial purpose of avoiding corrosion isachieved in a perfect manner. Corrosion does not occur in cooling pipescoated according to the invention.

[0018] Also, in the application of the coating according to theinvention, no adhesion of glass residues occurs. Thus, the expense forthe removal of these residues can be eliminated so that the cleaning ofthe crucible at the end of a melting or refining process is not aproblem.

[0019] An additional, advantageous effect of the invention is in thefollowing: the layer according to the invention is an excellent electricinsulator. It prevents electric arc-over between individual areas of thecrucible. This effect has a quite important economic significance. Itallows an enlargement of the crucible. When the crucible is larger andcan thus accommodate a higher volume, the refining or melting operationis more economical. Normally, a maximum crucible height is found whenthe induction voltages occurring in the cooling fingers, if they becometoo large, discharge via the melt. This leads to the formation of anelectric arc and to the destruction of the skull crucible. However, ifthe cooling fingers are coated with Teflon, then induction voltages canoccur before the formation of an electric arc begins.

[0020] In a practical experiment, the following parameters were present:

[0021] The crucible involved a standing skull crucible made of copperpipes with a wall thickness of 1 mm.

[0022] The coating consisted of a plastic containing high amounts offluorine.

[0023] The coating thickness amounted to 150μ.

[0024] The softening temperature of the plastic was 300° Celsius.

[0025] The decomposition temperature of the plastic was 450° Celsius.

[0026] The copper pipes were water-cooled.

[0027] In the crucible, phosphate glasses, fluorophosphate glasses andalkali zinc silicate glasses were melted.

[0028] The melting temperature was up to 1500° Celsius.

[0029] The coating thickness of the material must not be too large. Thereason for this is the following: plastic is indeed known to be a heatdamping material and thus prevents the flow of heat from one side to theother. If the coating is too thick, then this can make it so thatsufficient heat is no longer drawn off, from the melt located in thecontact area with the plastic, by the cooling medium flowing in themetal pipe. The area mentioned would then accommodate temperatures thatare above the decomposition temperature of the plastic. This is also thereason why, in general, plastic can not be used as a base material ofthe pipe instead of the metal. A composite made of an inner metal pipeand an outer plastic pipe with corresponding coordination of thethickness of the plastic layer with the operating conditions of theprocess is required. This applies especially, as mentioned above, to thetemperature of the melt prevalent in the process. In any case, a goodconducting material is thus required as a base material. Experience hasshown thus far that the coating thickness of the plastic must be a fewmillimeters at maximum, and generally less than 1 mm.

[0030] As a base material of the pipe, copper comes into considerationespecially as mentioned above, but also platinum, steel, pure metals,aluminum, or alloys made of these metals.

[0031] Devices according to the invention are especially suitable forthe following applications:

[0032] For the manufacture of alkali zinc silicate glasses;

[0033] For the manufacture of phosphate glasses;

[0034] For the manufacture of fluorophosphate glasses;

[0035] For the manufacture of lanthanum borate glasses;

[0036] For the manufacture of glasses that release corrosive gases suchas SO₂, SO₃, Cl₂, or HF during manufacturing;

[0037] For the manufacture of dimming glasses that contain sulfur,selenium and/or tellurium compounds;

[0038] For the manufacture of glasses that are mixed together in amanufacturing process with elementary halogens, especially chlorine, inorder to remove dissolved water, for example;

[0039] For the manufacture of oxide melts that crystallize when coolingoff;

[0040] For the manufacture of corrosive glasses.

1. Device for melting or refining inorganic substances, especiallyglass; 1.1 with many metal pipes which can be connected to a coolingmedium, and which lie next to each other in such a way that togetherthey form a container; 1.2 with a high-frequency coil for introducingenergy into the container contents; 1.3 the metal pipes are coated witha plastic having a decomposition temperature that is below thetemperature of the melt; 1.4 the cooling system is designed and arrangedin such a way that the temperature of the boundary layer of the meltdirectly surrounding the component is below the decompositiontemperature of the coating material.
 2. Device according to claim 1,characterized in that the coating has a thickness of 250μ or less. 3.Device according to claim 1 or 2, characterized in that the coatingmaterial is a plastic that contains fluorine.
 4. Device according toclaim 3, characterized in that the coating material containsfluoroalkoxy.
 5. Device according to one of the claims 1 to 4,characterized in that the base material of the metal pipe is copper.